Introduction

With the vigorous development of nuclear power industry, radioactive wastewater pollution has become one of the most serious environmental problems today. 90Sr and 137Cs are primary radionuclides in radioactive wastewater [1]. These radionuclides can cause environmental pollution and endanger human health seriously [2, 3]. Traditional treatment method applied to wastewater containing radionuclides such as ion exchange [4, 5], liquid/liquid extraction [6], membrane methods are often limited by their several shortcomings such as low selectivity and high cost. Zeolite and hydroxyapatite [7] show good cation exchange capacity and high selectivity in the disposal of radioactive waste liquid. However, in order to obtain larger volume reduction ratio, new alternative methods should be studied urgently. Biosorption method, as a new promising method for strontium and cesium disposal, has shown a series of advantages like high sorption capacity, low production costs and less secondary sludge [8,9,10]. Therefore, it is meaningful to investigate the biosorption methods of radioactive wastewater. Much of the previous studies on the adsorption of radionuclide by various kinds of adsorbents have focused on the adsorption of single metal ions. Since industrial wastewater contains many kinds of radionuclides, it is necessary to research the binary adsorption of two or more radionuclides and also to quantify the effect of the presence of an ion on the adsorption of another ion [11]. As a kind of engineering strain, Saccharomyces cerevisiae has been widely used in wastewater treatment [12]. In this study, we use irradiated Saccharomyces cerevisiae as a biological adsorbent to study its adsorption behavior in the individual and binary component solution containing Sr(II) and Cs(I) [13]. Due to the chemical properties of radionuclides and stable nuclides are similar, stable isotopes are used in the research. There is no available information in the previous literature for the simultaneous removal of Sr(II) and Cs(I) by irradiated Saccharomyces cerevisiae.

Herein, this paper aims to (i) research the feasibility of using irradiated Saccharomyces cerevisiae as an adsorbent for the individual and binary removal of Sr(II) and Cs(I) from aqueous solutions; (ii) investigating the effects of several environmental factors on Sr(II) and Cs(I) adsorption; (iii) Using different binary-isotherm models to describe the adsorption behavior in binary mixtures; (iv) The microstructure was observed by scanning electron microscope (SEM) and the composition was analyzed by energy dispersive spectrometer (EDS) method.

Materials and methods

Materials

Saccharomyces cerevisiae (CICC 30225) were obtained from the China Center of Industrial Culture Collection (CICC). Yeast were cultured in liquid media containing glucose (50.0 g), yeast extract (0.5 g), Na2HPO4 (0.5 g), (NH4)2SO4 (1.0 g), urea (1 g), and 1500 ml distilled water. The strain in the exponential growth phase with dose rate was 750 Gy h−1, at the Microbiology Laboratory of the Nuclear Science and Technology Department of Nanjing University of Aeronautics and Astronautics (NUAA). The initial dose was 0; 500; 1000; 2000; 3000 and 4000 Gy [14] and the final irradiated strains were obtained. We can obtain the preparation of stock solutions of strontium and cesium by the means of dissolving strontium nitrate (Sr(NO3)2) and cesium chloride (CsCl) with deionized water respectively.

Characterization

The radiation resistance and genetic stability of treated Saccharomyces cerevisiae were performed under irradiation condition (Dose: 4000 Gy). After irradiation, the samples were taken out and the survival under a microscope was observed by using methylene blue staining. The microscopic morphology and composition of irradiated Saccharomyces cerevisiae before and after adsorption was performed using scanning electron microscope (FEI quanta 650) and energy dispersive spectrometer (OXFORD X-act).

Methods

Researches of Sr(II) and Cs(I) adsorption on irradiated Saccharomyces cerevisiae was performed under ambient conditions via batch experiments. The desired concentrations of Sr(II) and Cs(I) was prepared by diluting the stock solution. The pH of diluted solution was adjusted by hydrochloric acid and sodium hydroxide solutions. Then, the irradiated Saccharomyces cerevisiae (cell wet weight: 100 mg) were mixed with diluted solution in the polyethylene tubes (10 mL). After a preset shaking time, the liquid phases were separated by centrifuge under 8000 rpm for 15 min. The supernatants were collected for analysis of the residual metal ion concentrations using Atomic Absorption Spectroscopy (AAS) (Perseeta5-990, China). All the experiments were performed in triplicate. The equilibrium adsorption capacity and adsorption rate of radionuclide was calculated using the following equation [15]:

$$q_{\text{e}} = \frac{{\left( {c_{0} - c_{\text{e}} } \right) \times V}}{M}$$
(1)
$$Q = \frac{{c_{0} - c_{\text{e}} }}{{c_{0} }} \times 100\%$$
(2)

where \(q_{\text{e}}\) is the equilibrium adsorption capacity of Saccharomyces cerevisiae (mg g−1), V is the volume of metal solution (L), M is the adsorbent dosage of Saccharomyces cerevisiae (g). Q is the equilibrium adsorption rate (%), \(c_{0}\) is the initial concentration of metal solution (mg L−1), \(c_{\text{e}}\) is the equilibrium concentration of metal solution (mg L−1).

Data analysis

The adsorption experimental data were analyzed by Origin9.1 with different isotherm models. The isotherm parameters of all the multi-component models can be found by minimizing Marquardt’s percent standard deviation (MPSD) [16]. MPSD value is used to evaluate the accuracy and adequacy [17, 18] of different isotherm model to fit the experimental data. MPSD is given as

$${\text{MPSD}} = 100\sqrt {\frac{1}{{n_{\text{m}} - n_{\text{p}} }}\mathop \sum \limits_{i = 1}^{n} \left( {\frac{{\left( {\mathop \sum \nolimits_{i = 1}^{N} q_{{{\text{e}},i,{ \exp }}} } \right) - \left( {\mathop \sum \nolimits_{i = 1}^{N} q_{{{\text{e}},i,{\text{cal}}}} } \right)}}{{\mathop \sum \nolimits_{i = 1}^{N} q_{{{\text{e}},i,{ \exp }}} }}} \right)_{i}^{2} }$$
(3)

\(n_{\text{m}}\) is the number of measured points, \(n_{\text{p}}\) is the number of unknown parameters in the adsorption isotherm model, \(q_{{{\text{e}},i,{ \exp }}}\) is the experimental value, \(q_{{{\text{e}},i,{\text{cal}}}}\) is the theoretical value. The smaller the MPSD value is the higher the accuracy and precision of the model used are and the smaller the error between the theoretical and experimental values is. MPSD values were analyzed by EXCEL 2010.

Results and discussion

Characterization

Figure 1a indicates that the treated Saccharomyces are not dyed blue. Figure 1b indicates that the untreated Saccharomyces cerevisiae are entirely dyed dark blue. Such results imply irradiated gamma-ray treated Saccharomyces can still maintain high activity in high radiation environment while almost all the untreated Saccharomyces cerevisiae died in high radiation environment. Therefore, gamma-ray treated Saccharomyces has high radiation resistance and genetic stability and we have successfully prepared irradiated Saccharomyces cerevisiae.

Fig. 1
figure 1

Survival of a gamma-ray treated and b untreated Saccharomyces cerevisiae exposed to irradiation environment. Dose = 4000 Gy, dose rate = 750 Gy h−1

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Effect of temperature

The effect of temperature on the adsorption for Sr(II) and Cs(I) onto irradiated Saccharomyces cerevisiae was carried out over the temperature range 298–312 K and the result are displayed in Fig. 2. The optimal adsorption rate (Q) and adsorption capacity (\(q_{\text{e}}\)) for Sr(II) and Cs(I) are 18.91 mg g−1 (94%) and 12.94 mg g−1 (64%), respectively when temperature is 305 K. As the temperature drops or rises at 305 K the adsorption rate and adsorption capacity for Sr(II) and Cs(I) decreased with the change of temperature.

Fig. 2
figure 2

Effects of temperature on the adsorption of Sr(II) and Cs(I) using irradiated Saccharomyces cerevisiae. C 0 = 200 mg L−1, M = 100 mg, time = 12 h, pH 7.0

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The possible reason is that some cell enzymes involved in adsorption were less active when the temperature decreased or increased at 305 K. So, we think that the optimum adsorption temperature for Cs(I) and Sr(II) is 305 K using irradiated Saccharomyces cerevisiae.

Effect of pH

The effect of pH on the adsorption for Sr(II) and Cs(I) onto irradiated Saccharomyces cerevisiae was carried out over the pH range 3.0–8.0 and the contact time was 12 h. The results are displayed in Fig. 3 [19, 20]. As shown in Fig. 3, in the case of Sr(II) adsorption optimum pH value was found at 5.0. The maximum adsorption rate (Q) and adsorption capacity (\(q_{\text{e}}\)) were 94% and 18.91 mg g−1 respectively. In the case of Cs(I) adsorption, optimum pH values were estimated to 6.0. The maximum adsorption rate (Q) and adsorption capacity (\(q_{\text{e}}\)) were 64% and 12.93 mg g−1 respectively. With the increase of pH value from 3 to 5, the adsorption rate and adsorption capacity for Sr(II) and Cs(I) increased gradually.

Fig. 3
figure 3

Effects of pH value on the adsorption of Sr(II) and Cs(I) using irradiated Saccharomyces cerevisiae. C 0 = 200 mg L−1, M = 100 mg, time = 12 h, T = 305 K

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This phenomenon may be that at lower pH protonation of the surface functional groups on the cell wall decreases the metal uptake of the biomass, whereas at increasing pH values, the increases of negative charge in the solution leads to the deprotonation of the metal binding sites [21]. Higher pH values were not studied because the metal forms hydroxide precipitation in alkaline conditions. The difference in optimum pH values may be attributed to the different metallic property [22]. Because yeast has better adsorption capacity both on Sr(II) and Cs(I) when pH is 5, the pH value of subsequent test is all 5.

Adsorption kinetics

The kinetic data of Sr(II) adsorption are shown in Fig. 4a. The kinetic data of Cs(I) adsorption are shown in Fig. 4b. As shown in Fig. 4a, b, the adsorption increases rapidly in the first 2 h [23], then changed slightly and entered the adsorption equilibrium period after 4 h. The kinetic models are used to explore the adsorption mechanism. The pseudo-first-order adsorption model and the pseudo-second-order one can be presented as follows:

Fig. 4
figure 4

Effects of time on a Sr(II) and b Cs(I) adsorption capacity using irradiated Saccharomyces cerevisiae and corresponding kinetic plots for adsorption of Sr(II) and Cs(I) ions. C 0 = 600 mg L−1, M = 100 mg, T = 305 K, pH 5.0

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$$\ln \left( {q_{\text{e}} - q_{\text{t}} } \right) = { \ln }q_{\text{e}} - k_{1} t$$
(4)
$$\frac{{{\text{d}}qt}}{{{\text{d}}t}} = k_{2} (q_{\text{e}} - q_{t} )^{2}$$
(5)

where k1 and k2 is the rate constant (g mg−1 h) and \(q_{t}\) is adsorption capacity at time t (mg g−1); \(q_{\text{e}}\) is equilibrium adsorption capacity (mg g−1).

The relative values can be seen in Table 1, the pseudo-second-order model have a greater correlation coefficient than pseudo-first-order adsorption model, suggesting that the adsorption for Sr(II) and Cs(I) can be better fitted by pseudo-second-order model. It indicates the adsorption process includes both external mass transfer and intraparticle diffusion (Table 2).

Table 1 The parameters of pseudo-first-order and pseudo-second-order models
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Table 2 The parameters of mono-adsorption isotherm modelling
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Mono-adsorption equilibria

The equilibrium adsorption capacity and adsorption rate for Sr(II) and Cs(I) as a function of initial ion concentration are shown in Fig. 5a, b respectively. The adsorption rate for Sr(II) and Cs(I) steeply decreases with the initial metal ions concentration up to 600 mg L−1 while the trend of adsorption capacity is opposite. In the case of Sr(II), there is a gradually decrease in the metal adsorption rate around 19.9% and increase in the adsorption capacity around 43.2 mg g−1 by altering initial Sr(II) concentration from 25 to 600 mg L−1. Comparing to Sr(II), The adsorption rate of Cs(I) decrease significantly around 51.2% with adsorption capacity increase around 14.7 mg g−1. It can be explained that the adsorption capacity increased with the increase of unit available binding sites while the adsorption rate decreased with the decrease of available binding sites of unit metal ions. To better discuss the adsorption mechanism, by the Langmuir, Freundlich and Redlich–Peterson isotherms models were used to simulate the experimental data.

Fig. 5
figure 5

Effects of initial concentration of a Sr(II) and b Cs(I) on the adsorption using irradiated Saccharomyces cerevisiae. M = 100 mg, T = 305 K, time = 4 h pH 5.0

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The theoretical basis of Langmuir equation relies on the assumption that the surface properties of adsorbent are homogeneous and a finite number of binding sites were distributed on the cell surface. The adsorption process of fluid molecules onto the solid surface is monolayer adsorption. Its expression is given by Eq. (2):

$$q_{\text{e}} = \frac{{q_{ \hbox{max} } K_{\text{L}} C_{\text{e}} }}{{1 + K_{\text{L}} C_{\text{e}} }}$$
(6)

where \(q_{ \hbox{max} }\) and \(K_{\text{L}}\) are the Langmuir coefficients. \(q_{ \hbox{max} }\) is the maximum adsorption capacity (mg g−1), and \(K_{\text{L}}\) is a constant related to the affinity of the binding sites. \(q_{ \hbox{max} }\) and \(K_{\text{L}}\) can be determined from the slope and intercept of \(C_{\text{e}} /q_{\text{e}}\) versus \(C_{\text{e}}\) linear plot.

The Freundlich isotherm model considers the surface is heterogeneity. Adsorbate is combined to the binding sites by multilayer adsorption. It is empirically expressed as follows:

$$q_{\text{e}} = K_{\text{F}} C_{\text{e}}^{1/n}$$
(7)

\(K_{\text{F}}\) (Lm g−1) is a parameter representing the adsorption capacity. Adsorption capacity is positively correlated with \(K_{\text{F}}\) value. n is the Freundlich adsorption constant known as biosorbent intensity. The model parameters of Freundlich isotherm can be determined from plotting lnq e versus ln C e.

The Redlich–Peterson isotherm contains three parameters that can be used to both homogenous and heterogeneous systems. It is empirically expressed as follows:

$$q_{\text{e}} = \frac{{K_{\text{R}} C_{\text{e}} }}{{1 + a_{\text{R}} C_{\text{e}}^{\beta } }}$$
(8)

this is hybrid isotherm incorporates three parameters into an empirical equation by combining both Langmuir and Freundlich isotherms.

Figure 6a, b present adsorption isotherms and the corresponding Langmuir, Freundlich and R–P plot for Sr(II) and Cs(I). Table 3 lists the related data fitted by these models. According to the R 2 values, Langmuir model simulate the adsorption experimental data of Sr(II) and Cs(I) better than other models, suggesting the monolayer adsorption for Sr(II) and Cs(I) with uniform adsorption energy. The adsorption capacity for Sr(II) on irradiated Saccharomyces cerevisiae is in the range of the previous reported yeast [13]. However, The adsorption capacity of Cs(I) on irradiated Saccharomyces cerevisiae have a significant difference compared with previous report [24]. The reason may be that the cell wall surface structure has changed by irradiation which produced an increase of Cs(I) affinity. The K L value of Sr(II) are higher than Cs(I) by Freundlich model showed Sr(II) easier to bind with cell surface than Cs(I). This is similar from previous studies [24]. Nieboer and Richardson had proposed a concept of covalent index [25], which is a complex parameter computed using the following expression: \(x_{\text{m}}^{2} r\).where \(x_{\text{m}}\) is electronegativity and r is ionic radius. Many researchers have found that the adsorption capacity is related to the covalent index. Previous papers have found an increasing trend between maximum adsorption capacities of yeast for metal ions and the covalent index of the element. Research shows the covalent index of Sr(II) was higher than Cs(I) [26], the results of this experiment accords with the theory of covalent index.

Fig. 6
figure 6

Adsorption isotherms and the corresponding Langmuir, Freundlich and R–P plot for a Sr(II) and b Cs(I) on irradiated Saccharomyces cerevisiae

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Table 3 Binary-component isotherm parameter values for the simultaneous adsorption of Sr(II) and Cs(I) by irradiated Saccharomyces cerevisiae
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Binary-adsorption equilibria

In binary ion mixture-irradiated Saccharomyces cerevisiae systems for each initial concentration of Sr(II) solution: viz 25, 75, 200,400 and 600 mg L−1, the Cs(I) concentration was varied in the range of 25–600 mg L−1 (viz 25, 75, 200, 400 and 600 mg L−1). Figure 7a presents the comparison of equilibrium adsorption isotherms of Sr(II) at varying concentrations of Cs(I). Figure 7b presents the comparison of equilibrium adsorption isotherms of Cs(I) at varying concentrations of Sr(II). With the initial Sr(II) concentration increases from 0 to 600 mg L−1, the equilibrium adsorption capacity range of Cs(I) using irradiated Saccharomyces cerevisiae changed from 2.0–16.6 to 0.7–6.4 mg g−1. With the initial Cs(I) concentration increases from 0 to 600 mg L−1, the equilibrium adsorption capacity range of Sr(II) using irradiated Saccharomyces cerevisiae changed from 2.4–45.6 mg g−1 to 1.7–30.3 mg g−1. The initial concentration provides the necessary driving force to overcome the resistances to the mass transfer of Sr(II) and Cs(I) between the aqueous and the solid phases. With the increases of initial metal concentration, the binding sites on the cell surface can adsorb more metal ions, thus increasing the adsorption capacity [27]. In general, there are three possible behaviors of adsorbate in a multi-component solution: synergism (The combined effect of each component in the solution is greater than the effect of a single component in the solution), antagonism (The combined effect of each component in the solution is less than the effect of a single component in the solution) and non-interaction (There was no interaction between the different components in the mixture). The combined effect of the binary mixture of Sr(II) and Cs(I) maybe antagonistic by analyzing Fig. 7. To better discuss the binary adsorption mechanism, by the Extended Langmuir, Non-modified Langmuir, Modified Langmuir, Non-modified R–P, Modified R–P and Sheindorf–Rebuhn–Sheintuch isotherms models were used to simulate the experimental data.

Fig. 7
figure 7

a Comparison of equilibrium adsorption isotherms of Sr(II) at varying concentrations of Cs(I). b Comparison of equilibrium adsorption isotherms of Cs(I) ion at varying concentrations of Sr(II). M = 100 mg, T = 305 K, time = 4 h, pH 5.0

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Non-modified competitive Langmuir model is the extension of the basic Langmuir model [28], It is empirically expressed as follows:

$$q_{{{\text{e}},i}} = \frac{{q_{{{\text{m}},i}} \;K_{{{\text{L}},i}} \;C_{{{\text{e}},i}} }}{{1 + \mathop \sum \nolimits_{j = 1}^{N} K_{{{\text{L}},j}} C_{{{\text{e}},j}} }}$$
(9)

where \(q_{{{\text{m}},i}}\) and \(K_{{{\text{L}},i}}\) are the Langmuir coefficients obtained by individual Langmuir model respectively. \(C_{{{\text{e}},i }}\) is the concentration of different components in binary solution.

However, \(q_{{{\text{m}},i}}\) and \(K_{{{\text{L}},i}}\) may not describe exactly the binary-component adsorption behavior of metal ion mixtures. For that reason, the adsorption behavior in multi-component solutions can be described more accurately by using the modified isotherms related to the individual isotherm parameters and correction factors. It is empirically expressed as follows:

$$Q_{{{\text{E}},i}} = \frac{{Q_{{{\text{M}},i}} K_{{{\text{L}},{\text{i}}}} (C_{{{\text{E}},i}} /_{{{\text{L}},i}} )}}{{1 + \mathop \sum \nolimits_{J = 1}^{N} K_{{{\text{L}},j}} (C_{{{\text{E}},i}} /_{{{\text{L}},i}} )}}$$
(10)

where interaction term, \(\eta_{{{\text{L}},i}}\) is a characteristic depends on each species and the concentrations of the other components. The modified competitive Langmuir isotherm is the extension of the competitive Langmuir model by adding the interaction term \(\eta_{{{\text{L}},i}}\).

Yang [29] raised the extended Langmuir isotherm for multi-component systems by expanding the mono-component Langmuir equation. The new equation assumed that the surface sites are uniform, and that all the adsorbate molecules (ions) in the solution compete for the same surface sites. It is empirically expressed as follows:

$$Q_{{{\text{E}},i}} = \frac{{Q_{ \hbox{max} } K_{{{\text{EL}},i}} C_{{{\text{E}},i}} }}{{1 + \mathop \sum \nolimits_{J = 1}^{N} K_{{{\text{EL}},j}} C_{{{\text{E}},j}} }}$$
(11)

Sheindorf et al. extended the Freundlich equation to give Sheindorf–Rebuhn–Sheintuch (SRS) equation for multi-component systems. It is empirically expressed as follows:

$$Q_{{{\text{E}},i}} = K_{{{\text{F}},i}} C_{{{\text{E}},i}} \left( {\mathop \sum \limits_{J = 1}^{N} A_{{{\text{I}}j}} C_{{{\text{E}},j}} } \right)^{{(1/N_{\text{I}} - 1)}}$$
(12)

the competition coefficients \(A_{{{\text{I}}j}}\) in the SRS model represents the inhibitory effect of component i on the adsorption of component j and can be determined from the thermodynamic data, or more likely, from the experimental sorption data of binary-component systems.

Similar to Modified Langmuir isotherm model, the competitive non-modified Redlich–Peterson model is modified, using an interaction term η R,i to the obtain modified competitive Redlich–Peterson model. Both is empirically expressed as follows:

$$Q_{{{\text{E}},i}} = \frac{{K_{{{\text{R}},i}} C_{{{\text{E}},i}} }}{{1 + \mathop \sum \nolimits_{J = 1}^{N} A_{{{\text{R}},j}} C_{{{\text{E}},j}}^{{{\text{\rm B}},j}} }}$$
(13)
$$Q_{{{\text{E}},i}} = \frac{{K_{{{\text{R}},i}} (C_{{{\text{E}},i}} /_{{{\text{R}},i}} )}}{{1 + \mathop \sum \nolimits_{J = 1}^{N} A_{{{\text{R}},j}} (C_{{{\text{E}},i}} /_{{{\text{R}},i}} )^{{{\text{\rm B}},j}} }} .$$
(14)

Table 3 lists the related data fitted by these binary isotherm models. In order to evaluate the accuracy and adequacy of different isotherm models intuitively, the comparisons of the experimental and calculated values of the equilibrium adsorption capacity on Sr(II) and Cs(I) are demonstrated by Fig. 8a, b respectively. The result simulated by the SRS model has a larger deviation from the 45° line, it indicate that SRS model (MPSD = 21.7) can’t be used to estimate the binary adsorption. It may imply that there are a finite number of binding sites which are inhomogeneous distributed over the adsorbent surface and there is no exponential distribution of site adsorption energies for each component. Reverse, the modified Langmuir model can describe the binary adsorption accurately (MPSD = 6.2). The Ki values of Sr(II) and Cs(I) in the modified Langmuir model are 1.132 and 1.590 respectively. It indicate that the adsorption for Cs(I) was strongly affected by the presence of Sr(II), while the adsorption for Sr(II) was less affected by the presence of Cs(I). According to the MPSD values, modified Langmuir model simulate the experimental data better than non-modified Langmuir model (MPSD = 9.8), suggesting that there is mutual inhibition between the binary-component solutions. The total adsorption capacity (\(q_{ \hbox{max} }\)) by irradiated Saccharomyces cerevisiae is 43.3 mg g−1. This value is considerably lower than the sum of the adsorption capacity for Cs(I) and Sr(II) resulting from the individual component adsorption. For that reason, the adsorption sites for Sr(II) and Cs(I) in the binary system onto irradiated Saccharomyces cerevisiae may likely be partially overlapped. This may also imply that a variety of different binding sites exist on the surface of irradiated Saccharomyces cerevisiae, which are partially specific to individual ions. Three-dimensional (3D) adsorption isotherm surfaces are used to evaluate the performance of the binary metal ions adsorption system [30, 31]. A 3D graphical representation of the adsorption isotherm plot for the binary-component adsorption system is given in Fig. 9. In this plot, the experimental data points are shown along with the predicted isotherms surface using the modified Langmuir model.

Fig. 8
figure 8

Comparison of the experimental and calculated \(q_{\text{e}}\) values of a Sr(II) and b Cs(I) in a binary mixture of Sr(II) and Cs(I)

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Fig. 9
figure 9

Binary adsorption isotherms and the corresponding Modified Langmuir surface plot for a Sr(II) and b Cs(I) on irradiated Saccharomyces cerevisiae

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SEM and EDS analysis

The microstructure before and after adsorption for Sr(II) and Cs(I) by irradiated Saccharomyces cerevisiae was observed by SEM (Fig. 10). Figure 10a shows the smooth cell surface. Figure 10b, c indicate that different crystals formed on the cell surface when exposure to Sr(II) and Cs(I) respectively. The cell surface has formed long strips crystalline when exposure to Sr(II). The cell surface has formed a granular crystallization when exposure to Cs(I). The composition of cell before exposure to Sr(II) and Cs(I) are shown in Fig. 11a. The composition of the crystalline formed on cell surface after exposure to Sr(II) and Cs(I) are shown in Fig. 11b, c. The cell wall contains mainly C, H, O elements. The crystals formed on the cell surface are precipitate of Sr(II) and Cs(I), respectively.

Fig. 10
figure 10

SEM images of surface morphology for irradiated Saccharomyces cerevisiae before (a) and after adsorption on b Sr(II) ion and c Cs(I) ion

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Fig. 11
figure 11

The EDS results a before and after exposure to b Sr(II) and c Cs(I) ions by irradiated Saccharomyces cerevisiae

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Conclusion

In this research, the characterization of irradiated Saccharomyces cerevisiae was detected by using methylene blue staining, SEM and EDS. The results of methylene blue staining indicate that the gamma-ray treated yeast has strong radiation resistance. The effects of different environmental factors on the adsorption for Sr(II) and Cs(I) using irradiated Saccharomyces cerevisiae were conducted by employing batch experiments. The obvious effect of temperature on Sr(II) and Cs(I) adsorption indicates that some cell enzymes involved in adsorption. The effect of pH on Sr(II) and Cs(I) adsorption shows protonation of the surface functional groups at lower pH value and deprotonation of the metal binding sites at higher pH value. The kinetics results show that the adsorption process includes both external mass transfer and intraparticle diffusion. In the case of Mono-adsorption equilibria, the single adsorption for Sr(II) and Cs(I) using irradiated Saccharomyces cerevisiae can be simulated by the Langmuir and Redlich–Peterson isotherm model very well and the adsorption process is chemical adsorption. The adsorption capacity for Sr(II) was significantly higher than that for Cs(I) on irradiated Saccharomyces cerevisiae. This result accords with theory of covalent index. In the case of Mono-adsorption equilibria, the binary adsorption for Sr(II) and Cs(I) in binary mixtures can be simulated by the modified Langmuir model accurately. The combined effect of the binary mixture of Sr(II) and Cs(I) may be antagonistic. SEM images shows that different crystals formed on the cell surface when exposure to Sr(II) and Cs(I) respectively. EDS analysis indicates the crystals are precipitation contains Sr(II) and Cs(I) respectively. The results provide comprehensive information for treatment of liquid radioactive waste. It has great reference value for the future study of multivariate adsorption.